Current optical networks are engineered to handle amplifier noise and chromatic dispersion. Polarization Mode Dispersion (PMD) is a phenomenon that occurs due to splitting of energy of an optical pulse propagating in a fiber into two polarization modes. Since these two modes have slightly different refractive indices, the two modes travel at different velocities and thus, PMD results in pulse spreading. PMD compensation (PMDC) is a difficult and expensive task and, hence, fiber links with high PMD coefficients are largely avoided for high speed transmission. However, if there are no other available options for dealing with PMD than to use a high PMD link, the operator has to pay a high price for deploying costly regenerators designed to zero out the dispersion.
Due to the difficulty and high cost associated with PMDC, only a few commercial systems have been deployed. Ten Gigabit per seconds (10 Gbps) transmission is fairly tolerant to PMD because of its long, 100 picoseconds (ps), symbol period. A general rule of thumb in optical network engineering is that having a PMD level up to a third of the symbol period can be tolerated. A third of the symbol period for a 10 Gbps transmission translates to having the symbol period set at 33 ps. Hence, network operators engineer their networks to keep the PMD below this level.
Higher rate channels are making their way into networks initially designed for 10 Gbps. These higher rate services of 40 Gbps and someday 100 Gbps have shorter signaling periods and, as such, are much more susceptible to PMD. For this reason and also to solve the problem of lower rates, such as 10 Gbps, on a high PMD fiber, there is a need to compensate for PMD.